Method for producing a low-cost hypereutectoid bearing steel

ABSTRACT

METHOD FOR PRODUCING A LOW-COST HYPEREUTECTOID BEARING STEEL HAVING CHARACTERISTICS EQUAL TO OR SUPERIOR TO HIGHER PRICED STEELS OF THIS TYPE AND CONTAINING AS ESSENTIAL ALLOYING ADDITIONS .80-1.10% CARBON, .50-1.00% MANGANESE, .20-.60% CHROMIUM AND .05-.25% MOLYBDENUM. A STEEL OF THIS TYPE, USUALLY IN THE FORM OF WIRE, ROD, BAR, FORGINGS, RINGS OR TUBING, IS ANNEALED AT A MAXIMUM TEMPERATURE OF 1400*F. FOR THRITEEN HOURS TO PRODUCE AN ANNEALED MICROSTRUCTURE COMPRISING SPHEROIDIZED CARBIDES IN A MATRIX OF FERRITE. AFTER FORMING INTO BEARING RACES OR BALLS, THE FORMED PRODUCT IS QUENCHED FROM 1550*F. AND THEREAFTER TEMPERED TO PRODUCE A STEEL HAVING A ROCKWELL C HARDNESS OF ABOUT 60 OR GREATER AND EXCELLENT FATIGUE STRENGTH.

c. P. WEIG EL 3,704,183

METHOD FOR PRODUCING A LOW-COST HYPEREUTECTOID BEARING! STEEL Nov. 28,I972 Filed May 26, 1971 m mm INVYENTOR. CHARLES I? WEIGEL A! rarnoysUnited States Patent O ice 3,704,183 METHOD FOR PRODUCING A LOW-COSTHYPEREUTECTOID BEARING STEEL Charles P. Weigel, Canton, Ohio, assignorto The Timken Company, Canton, Ohio Filed May 26, 1971, Ser. No. 146,911Int. Cl. B22d 25/ C21d 7/14 US. Cl. 148-126 2 Claims ABSTRACT OF THEDISCLOSURE Method for producing a low-cost hypereutectoid bearing steelhaving characteristics equal to or superior to higher priced steels ofthis type and containing as essential alloying additions .801.l0%carbon, .50l.00% manganese, .20.60% chromium and .05.25% molybdenum. Asteel of this type, usually in the form of wire, rod, bar, forgings,rings or tubing, is annealed at a maximum temperature of 1400 F. forthirteen hours to produce an annealed microstructure comprisingspheroidized carbides in a matrix of ferrite. After forming into bearingraces or balls, the formed product is quenched from 1550 F. andthereafter tempered to produce a steel having a Rockwell C hardness ofabout 60 or greater and excellent fatigue strength.

BACKGROUND OF THE INVENTION As is known, AISI 52100 steel has been usedextensively, and almost exclusively, for ball bearing applications. Sucha steel contains as essential alloying additions, .98-1.10% carbon,.25-.45% manganese and 1.30-1.60% chromium. The relatively high amountof chromium is added to assist in hardenability; however this materiallyincreases the cost of the product.

In the manufacture of bearings from AISI 52100 steel, tubular shapes,bars, forgings and rings (for forming bearing races) and wire or rod(for forming balls) are initially annealed at about 1450 F., maximumtemperature, and furnace cooled at a controlled rate for twenty-twohours, followed by air cooling. In the heat treating process,spheroidized carbides are formed in a matrix of ferrite to produce aproduct having good machinability, cold heading and/or cold formingproperties. After the machining, cold heading or forming operation, theballs and races are quenched in oil from about 1550 F. and then temperedto achieve a Rockwell C hardness of 60 or greater. Of course, the steelmust also have good fatigue strength.

While various alloys with about .6 or .7% carbon have been proposed inthe past as low-cost substitutes for AISI 52100 bearing steel, thesegenerally result in greater difiiculty in obtaining spheroidizedannealed microstructures or fatigue strengths inferior to those of AISI52100.

SUMMARY OF THE INVENTION The present invention resides in the discoverythat a high carbon bearing steel can be prepared which containsubstantially less total alloy content than conventional bearing steels,without sacrificing the hardenability, fatigue strength and processingcharacteristics of higher alloy bearing steels. At the same time, theannealing time required to achieve adequate carbon diffusion to producea spheroidized microstructure is materially reduced with the steel ofthe invention.

Specifically, it has been found that the carbon content Patented Nov.28, 1972 DESCRIPTION OF THE PREFERRED EMBODIMENTS The bearing steel ofthe invention has the following broad and preferred compositions:

Analysis, percent by Welg 11 Broad Preferred Carbon -1.10 .90 Manganese50-1. 00 50-. 80 Sulfur 005-. 06 008. 04 Silicon. 20-. 35 .20. 35Chromiu 20-. 60 25-. 40 Molybdenu 05-. 25 05-. 10 Nickel .35 .35 IronBalance Balance 1 Maximum.

Nickel and silicon are relatively unimportant in achieving the desiredcharacteristics of the alloy and can be classified as residual amounts..20-.35% silicon is that amount found in most alloy steels. Similarly, acertain amount of sulfur is found in every steel; however, it is desiredto deliberately include .008-.04% sulfur to improve machinability andfatigue strength. As was mentioned above, carbon, manganese, chromiumand molybdenum are the essential additions, the chromium and molybdenumacting to increase hardenability. In the preferred analysis given above,percent by Weight of carbon is however it should be understood that thiscan vary plus or minus five points, in accordance with usualmetallurgical practices.

The steel of the foregoing composition is melted, formed into tubing,forgings, rings, bars, rod, wire or other shapes, and then annealed at amaximum temperature of 1400 F. for thirteen hours in order to achieve aspheroidized microstructure comprising spheroidized carbides in a matrixof ferrite. In the annealing step, the steel is passed through elevenzones of a roller hearth furance at a constant speed, the time in eachzone being about oneeleventh of the total time of thirteen hours. Thetemperatures of the respective zones are shown in the following Table I:

TABLE I.ANNEALING TEMPERATURES IN ELEVEN ZONE ROLLER HEARTH FURNACE Zonel 2 3 4 5 Temperature, F

with the required annealing time of twenty-two hours at v,

3 a maximum temperature of 1450" F. for AISI 52100 steel. Furthermore,the shorter annealing time reduces the cost of the product and resultsin less scaling and less decarburization.

The annealed product is then cold headed or cold formed into balls andmachined or formed into races which are thereafter heated to atemperature of 1550 F., followed by quenching in oil. In this condition,the product has a Rockwell C hardness of about 63-66. The final step inthe process is to temper the product at 300 F. for about 1 to 2 hours toachieve a Rockwell C hardness of 60 or greater.

The resulting product has a density of 0.283 pound per cubic inch, aspecific gravity of 7.83, a modulus of elasticity of 29 l0 pounds persquare inch and a modulus of rigidity of 12x10 pounds per square inch.In addition, the product has excellent fatigue strength, comparable tothat of the higher alloy AISI 52100 bearing steel and much better thanAISI 5160 which is sometimes used as a low-cost substitute for hearingsteels. This is shown in the accompanying drawing wherein curve 10 isthat for the steel of the invention; whereas curve 12 is that obtainedfrom fatigue tests conducted on AISI 5160 steel. The steel of theinvention has an L fatigue life (i.e., a probability of 10% failures, or90% survival) after about 10 million stress cycles; whereas AISI 5160steel has an L fatigue life after only 6.5 million stress cycles. Notethat the improvement in fatigue life persists at 50% survival. Thus, thesteel of the invention has an L fatigue life of 45 million stress cyclesas compared with 25 million stress cycles for AISI 5160.

Typical hardenability of the steel of the invention, determined with theJominy hardenability test, is shown in the following Table II:

TABLE II.'IYPICAL HARDENABILITY (NO RMALIZE 1,656

F., ANNEAL, QUENOH 1,ao F.)

J distance from quenched end in) Me inch Rockwell C hardnessatcorrespending point 65 65 65 61 47 43 43 43 43 41 Norm-Minimumhardness on 0.5 inch section heated to 1,525 F.=l= for 20 minutes andoil quenched, 63.0 Rockwell C.

The effect of the austenitizing temperature on the steel of theinvention is shown in Table III:

TABLE IIL-HEAT TREATED PROPERTIES EFFECT OF AUSTENITIZING TEMPERATURENote that the austenitizing temperature has very little effect on theas-quenched hardness. Finally, the eifect of the tempering temperatureon the steel of the invention is shown in Table IV:

TABLE IV.-EFFECT OF TEMPERDNG TEMPERATURE (1,550 F. QUENOH) HardnessRockwell C after- Iempering One- Twotemperature hour hour F.) l tempertemper As-quenehed hardness; 65.0 Rockwell 0.

tion.

I claim as my invention:

1. In the method for forming a bearing element from an alloy steelconsisting essentially of .80-1.10% carbon, .50l.00% manganese, .20.60%chromium and .05.25% molybdenum, the steps of: melting said alloy steeland forming it into a desired shape, annealing said alloy steel at amaximum temperature of 1400 F. for thirteen hours to produce aspheroiclized microstructure comprising spheroidized carbides in amatrix of ferrite, thereafter heating said article to a temperature ofabout 1550" F. and quenching, and finally tempering said article at atemperature of about 300 F. to achieve a Rockwell C hardness of about atleast 60.

2. The method of claim 1 wherein said steel is annealed starting at atemperature of 1250 F. which increases during said thirteen hourannealing time to 1400 F. and thereafter decreases to a low of 1300 F.during said annealing time.

References Cited UNITED STATES PATENTS V HYLAND BIZOT, Primary ExaminerU.S. Cl. X.R.

